The Collaborative International Dictionary of English v.0.48:

Insect \In"sect\, a.
   1. Of or pertaining to an insect or insects.
      [1913 Webster]

   2. Like an insect; small; mean; ephemeral.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Insect \In"sect\ ([i^]n"s[e^]kt), n. [F. insecte, L. insectum,
   fr. insectus, p. p. of insecare to cut in. See Section. The
   name was originally given to certain small animals, whose
   bodies appear cut in, or almost divided. Cf. Entomology.]
   1. (Zool.) One of the Insecta; esp., one of the Hexapoda. See
      Insecta.
      [1913 Webster]

   Note: The hexapod insects pass through three stages during
         their growth, viz., the larva, pupa, and imago or
         adult, but in some of the orders the larva differs
         little from the imago, except in lacking wings, and the
         active pupa is very much like the larva, except in
         having rudiments of wings. In the higher orders, the
         larva is usually a grub, maggot, or caterpillar,
         totally unlike the adult, while the pupa is very
         different from both larva and imago and is inactive,
         taking no food.
         [1913 Webster]

   2. (Zool.) Any air-breathing arthropod, as a spider or
      scorpion.
      [1913 Webster]

   3. (Zool.) Any small crustacean. In a wider sense, the word
      is often loosely applied to various small invertebrates.
      [1913 Webster]

   4. Fig.: Any small, trivial, or contemptible person or thing.
      --Thomson.
      [1913 Webster]

   Insect powder,a powder used for the extermination of
      insects; esp., the powdered flowers of certain species of
      Pyrethrum, a genus now merged in Chrysanthemum. Called
      also Persian powder.
      [1913 Webster]

WordNet (r) 3.0 (2006):

insect
    n 1: small air-breathing arthropod
    2: a person who has a nasty or unethical character undeserving
       of respect [syn: worm, louse, insect, dirt ball]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "insect":
   Chilopoda, Chordata, Echiuroidea, Ectoprocta, Entoprocta, Lepisma,
   Monoplacophora, Nemertinea, Phoronidea, animal, ant, aphid, aphis,
   arachnid, arthropod, assassin bug, beast, bedbug, bee, beetle,
   bluebottle, borer, bug, butterfly, caddis fly, caterpillar,
   centipede, chafer, chigoe, chilopod, cicada, cicala, cockchafer,
   cockroach, crane fly, cricket, cucumber flea beetle, cur, curculio,
   daddy longlegs, damselfly, deer fly, diplopod, dobson fly, dog,
   dragonfly, drosophila, dung beetle, earwig, ephemerid, firefly,
   flea, fly, gadfly, glowworm, gnat, grain weevil, grasshopper,
   harvestman, hexapod, hornet, hound, hyena, jigger, katydid,
   kissing bug, lacewing, lady beetle, ladybird, ladybug, lantern fly,
   larva, locust, louse, maggot, mantis, mayfly, mealworm, midge,
   millepede, miller, millipede, mite, mole cricket, mongrel,
   mosquito, moth, nymph, pig, polecat, reptile, rice weevil, roach,
   scarab, scorpion, serpent, silverfish, skunk, snake, spider,
   springtail, squash bug, stag beetle, stinkbug, stone fly, swine,
   syrphus fly, tarantula, termite, tick, tiger moth, tsetse fly,
   tumblebug, varmint, vermin, viper, walkingstick, wasp, water bug,
   weevil, whelp, wood tick, worm, yellow jacket